An air conditioner includes a refrigerant circuit having: a compressor; an outdoor heat exchanger; an expansion valve; and an indoor heat exchanger all of which are connected in the stated order. In heating operation, the outdoor heat exchanger functions as an evaporator, and the indoor heat exchanger functions as a condenser. The refrigerant circuit provides a heating cycle in which the refrigerant circulates in the order of the compressor, the indoor heat exchanger, the expansion valve, and the outdoor heat exchanger.
In the heating cycle, outdoor air is cooled by the refrigerant in the outdoor heat exchanger, such that the outdoor heat exchanger can be frosted. To overcome the problem, Patent Document 1 discloses the following technique: when frosting of an outdoor heat exchanger is detected, the technique allows the rotation speed of a compressor to drop while heating operation is maintained, and keeps the outdoor heat exchanger from further frost.